System for providing fluid flow to nerve tissues

ABSTRACT

Provided is an apparatus that includes a nerve conduit and a nested manifold for providing a reduced pressure. Also provided is a system that includes a source of reduced pressure, a nerve conduit and nested manifold, and a conduit for providing fluid communication between the manifold and the source of reduced pressure. Additionally provided is a method that includes implanting the above nerve conduit and manifold at a site of damaged nerve tissue and applying a reduced pressure to the manifold thereby stimulating repair or regrowth of nerve tissue.

This application claims priority to U.S. Provisional Application Nos.61/142,053 and 61/142,065, each filed on Dec. 31, 2008. This applicationalso claims priority to U.S. Provisional Application No. 61/234,692,filed on Aug. 18, 2009 and U.S. Provisional Application No. 61/238,770,filed on Sep. 1, 2009. Each of the foregoing applications isincorporated herein by reference in their entirety.

BACKGROUND

1. Field of the Invention

The present application relates generally to tissue engineering and inparticular to apparatuses and systems suitable for use in treatment ofdamaged nerve tissue.

2. Description of Related Art

Clinical studies and practice have shown that providing a reducedpressure in proximity to a tissue site augments and accelerates thegrowth of new tissue at the tissue site. The applications of thisphenomenon are numerous, but application of reduced pressure has beenparticularly successful in treating wounds. This treatment (frequentlyreferred to in the medical community as “negative pressure woundtherapy,” “reduced pressure therapy,” or “vacuum therapy”) provides anumber of benefits, including faster healing and increased formation ofgranulation tissue. Typically, reduced pressure has been applied totissue through a porous pad or other manifolding device. The porous padcontains pores that are capable of distributing reduced pressure to thetissue and channeling fluids that are drawn from the tissue. The porouspad often is incorporated into a dressing having other components thatfacilitate treatment. A scaffold can also be placed into a defect tosupport tissue growth into the defect. The scaffold is usuallybioabsorbable, leaving new tissue in its place.

Scaffolds for reduced pressure treatment are described in, e.g.,WO08/091,521, WO07/092,397, WO07/196,590, WO07/106,594. The adequacy ofcurrent scaffolds for reduced pressure treatment can be evaluated inlight of current knowledge of wound healing. Injury to body tissuesresults in a wound healing response with sequential stages of healingthat include hemostasis (seconds to hours), inflammation (hours todays), repair (days to weeks), and remodeling (weeks to months). A highlevel of homology exists across most tissue types with regards to theearly phases of the wound healing process. However, the stages ofhealing for various tissues begin to diverge as time passes, with theinvolvement of different types of growth factors, cytokines, and cells.The later stages of the wound healing response are dependent upon theprevious stages, with increasing complexity in the temporal patterningof and interrelationships between each component of the response.

Strategies to facilitate normal repair, regeneration, and restoration offunction for damaged tissues have focused on methods to support andaugment particular steps within this healing response, especially thelatter aspects of it. To this end, growth factors, cytokines,extracellular matrix (ECM) analogs, exogenous cells and variousscaffolding technologies have been applied alone or in combination withone another. Although some level of success has been achieved using thisapproach, several key challenges remain. One main challenge is that thetiming and coordinated influence of each cytokine and growth factorwithin the wound healing response complicate the ability to addindividual exogenous factors at the proper time and in the correctcoordination pattern. The introduction of exogenous cells also facesadditional complications due to their potential immunogenicity as wellas difficulties in maintaining cell viability.

Synthetic and biologic scaffolds have been utilized to providethree-dimensional frameworks for augmenting endogenous cell attachment,migration, and colonization. To date nearly all scaffolds have beendesigned with the idea that they can be made to work with the biology.Traditional scaffolding technologies, however, rely on the passiveinflux of endogenous proteins, cytokines, growth factors, and cells intothe interstitium of the porous scaffold. As such, the colonization ofendogenous cells into the scaffold is limited by the distance away fromvascular elements, which provide nutrient support within a diffusionlimit of the scaffold, regardless of tissue type. In addition, thescaffolds can also elicit an immunogenic or foreign body response thatleads to an elongated repair process and formation of a fibrous capsulearound the implant. Taken together, these complications can all lead toless than functional tissue regeneration at the injury site.

It would therefore be advantageous to provide additional systems for therepair and remodeling of specialized tissues. The present inventionprovides such systems.

SUMMARY

The apparatuses, systems and methods of the illustrative embodimentsdescribed herein provide active guidance of nerve tissue repair andregeneration through an implanted manifold and nerve conduit. In oneembodiment, an apparatus for providing reduced pressure therapy andfacilitating growth of nerve tissue in a patient is provided thatincludes a nerve conduit and nested manifold adaptable for implantationat a damaged nerve site, wherein the manifold provides and distributesreduced pressure at the site of damaged nerve tissue. A manifoldaccording to the invention may also be coupled to a scaffold materialwhich further distributes reduced pressure and provides a structuralmatrix for growth of the tissue.

In another embodiment, a system for providing reduced pressure therapyand facilitating growth of nerve tissue in a patient is provided thatcomprises a source of reduced pressure for supplying reduced pressureand a manifold nested in a nerve conduit adaptable for implantation atthe tissue site, where the manifold is in fluid communication with thesource of reduced pressure. Such a system may also comprise a scaffoldmaterial coupled to the manifold which further distributes reducedpressure and provides a structural matrix for the growth of nervetissue. In a further embodiment, such a system may further comprise acanister for fluid capture and/or a valve for control of reducedpressure in fluid communication with, and positioned between, themanifold and the reduced pressure source. In still a further embodiment,a system according to the invention further comprises a fluid source influid communication with the manifold and the damaged nerve tissue.

In a further embodiment, a method of providing reduced pressure therapyand facilitating growth of nerve tissue at a site of nerve tissue damagein a patient is provided that includes implanting a nerve conduit andnested manifold at the tissue site, where the manifold provides areduced pressure to the damaged nerve tissue. The manifold may also becoupled to a scaffold material, wherein the scaffold material provides astructural matrix for the growth of the nerve tissue. In certainembodiments, the method further comprises providing a manifold in fluidcommunication with a fluid source, wherein the fluid source may be usedto deliver a fluid to the manifold and the damaged nerve tissue. In yeta further embodiment, the fluid source may comprise a fluid comprisingone or more bioactive compounds including, but not limited to, anantibiotic, an antiviral, a cytokine, a chemokine, an antibody, and agrowth factor.

Other objects, features, and advantages of the illustrative embodimentswill become apparent with reference to the drawings and detaileddescription that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, perspective view of a reduced-pressure system forrepairing a severed nerve including a nerve conduit and a firstembodiment of a manifold and a scaffold having fiber guides with asection of the nerve conduit removed to show the manifold and scaffold;

FIG. 2 is a schematic, perspective view of a reduced-pressure system offor repairing a severed nerve including a nerve conduit and a secondembodiment of a manifold and scaffold having fiber guides with a sectionof the nerve conduit removed to show the manifold;

FIG. 3 is a schematic, perspective view of the scaffold and fiber guidesof the reduced-pressure systems shown in FIGS. 1 and 2;

FIG. 4 is a schematic, side view of three embodiments of the fiberguides shown in FIG. 3;

FIG. 5 is a schematic, perspective view of a fourth embodiment of thefiber guides shown in FIG. 3;

FIG. 6 is a schematic, perspective view of the system in FIGS. 1 and 2showing the nerve conduit enclosing the damaged nerve; and

FIG. 7 is a schematic view of a fluid control system for the systemshown in FIGS. 1 and 2.

DETAILED DESCRIPTION

In the following detailed description of the illustrative embodiments,reference is made to the accompanying drawings that form a part hereof.These embodiments are described in sufficient detail to enable thoseskilled in the art to practice the invention, and it is understood thatother embodiments may be utilized and that logical structural,mechanical, electrical, and chemical changes may be made withoutdeparting from the spirit or scope of the invention. To avoid detail notnecessary to enable those skilled in the art to practice the embodimentsdescribed herein, the description may omit certain information known tothose skilled in the art. The following detailed description is,therefore, not to be taken in a limiting sense, and the scope of theillustrative embodiments are defined only by the appended claims.

Referring to FIG. 1, a reduced pressure therapy system 100 for applyingreduced pressure at a tissue site in the body of a patient to repair adefect such as, for example, a damaged nerve is disclosed. The damagednerve may have been pinched, partially disconnected or severed, orpartially degenerated as a result of disease. For example, the damagednerve in FIG. 1 is a severed nerve 102 having a proximal segment 104 anda distal segment 106 relative to the central nervous system (CNS) of thepatient. The severed nerve 102 has been damaged at a nerve damage site108 that has been severed or degenerated. The severed nerve 102 may bebranched or unbranched at the nerve damage site 108. The term “nervedamage site” as used herein refers to a wound or defect located on orwithin any nerve tissue, including, but not limited to, a disconnectedor partially disconnected nerve, a degenerated or partially degeneratednerve, and a compressed or pinched nerve. For example, reduced pressuretissue treatment may be used to enhance repair or regrowth of existingnerve tissue or to facilitate growth or grafted or transplanted nervetissue and/or cells.

The reduced pressure therapy system 100 comprises a nerve conduit 110that surrounds the severed nerve 102 at the nerve damage site 108 with asection of the nerve conduit 110 removed to show the nerve damage site108. The nerve conduit 110 is substantially tubular in shape and closesthe nerve damage site 108 and a portion of the proximal segment 104 andthe distal segment 106 that has not been damaged. The nerve conduit 110has an inner surface 112 that forms a nerve gap 114 with the surface ofthe nerve damage site 108, i.e., a luminal space between the innersurface 112 of the nerve conduit 110 and the surface of the nerve damagesite 108. The reduced pressure therapy system 100 also comprises areduced pressure source 115 for providing a reduced pressure and amanifold 120 fluidly coupled to the reduced pressure source 115 via afirst conduit 125. The manifold 120 is generally tubular or cylindricalin shape (see, e.g., the manifold disclosed in copending U.S. patentapplication Ser. No. 12/648,463, incorporated herein by reference) andpositioned within the nerve gap 114. The manifold 120 may have a varietyof shapes depending on the type of nerve damage, and in this particularembodiment has a tubular shape to occupy a portion of the nerve gap 114.The manifold 120 may also contain scaffold structure 134 that providesstructure for tissue growth and repair. The reduced pressure therapysystem 100 further comprises a canister 130 fluidly coupled between thereduced pressure source 115 and the manifold 120 via the first conduit125 to collect bodily fluids such as blood or exudate that are drawnfrom the scaffold structure 134 and the nerve gap 114. In oneembodiment, the reduced pressure source 115 and the canister 130 areintegrated into a single housing structure.

A further embodiment of a reduced-pressure system 200 is shown in FIG. 2and is substantially similar to the reduced-pressure system 100. Thereduced pressure therapy system 200 comprises a nerve conduit 110 thatsurrounds the severed nerve 102 at the nerve damage site 108 with asection of the nerve conduit 110 removed to show the nerve damage site108. The nerve conduit 110 is substantially tubular in shape and closesthe nerve damage site 108 and a portion of the proximal segment 104 andthe distal segment 106 that has not been damaged. The nerve conduit 110has an exterior surface 113 and an inner surface 112 that forms a nervegap 114 with the surface of the nerve damage site 108, i.e., a luminalspace between the inner surface 112 of the nerve conduit 110 and thesurface of the nerve damage site 108. The reduced-pressure system 200also comprises a reduced pressure source 115 for providing a reducedpressure and a manifold 220 fluidly coupled to the pressure source 115via a first conduit 125. The manifold 220 (see, e.g., the manifolddisclosed in copending U.S. patent application Ser. No. 12/648,458,incorporated herein by reference) is contained within a manifold chamber221 having a flange 222 extending from one end of the manifold chamber221 for securing the manifold chamber 221 to the nerve conduit 110. Theother end of the manifold chamber 221 is connected to the first conduit125 so that the manifold 220 is held in fluid communication with thefirst conduit 125. The manifold chamber 221 may be constructed of anybiocompatible material that is substantially impermeable to preserve themanifold's 220 fluid communication between the nerve gap 114 and thefirst conduit 125. The manifold chamber 221 is secured to the nerveconduit 110 by the flange 222 such that the manifold 220 is in fluidcommunication with the nerve gap 114 surrounding the surface of thenerve damage site 108, but positioned outside of the nerve gap 114. Incertain aspects the flange 222 is secured to the nerve conduit 110 withan adhesive. Moreover, in some applications, the flange 222 isdetachably secured to the nerve conduit 110 such that the flange 222 andmanifold chamber 221 can be removed from the nerve conduit 110 afterreduced pressure therapy is complete. In one embodiment, the manifold220 extends through the wall of the nerve conduit 110 in direct fluidcontact with the nerve gap 114. In another embodiment, where the nerveconduit 110 is porous, the flange 222 is secured to the exterior surface113 of the nerve conduit 110 so that the manifold 220 is positionedadjacent to the exterior surface 113 to be in fluid communication withthe nerve gap 114 via the porous wall of the nerve conduit 110.

As used herein, the term “coupled” includes direct coupling or indirectcoupling via a separate object. The term “coupled” also encompasses twoor more components that are continuous with one another by virtue ofeach of the components formed from the same piece of material. Also, theterm “coupled” may include chemical, mechanical, thermal, or electricalcoupling. Fluid coupling means that fluid is in communication withdesignated parts or locations.

In the context of this specification, the term “reduced pressure”generally refers to a pressure that is less than the ambient pressure ata tissue site that is subjected to treatment. In most cases, thisreduced pressure will be less than the atmospheric pressure of thelocation at which the patient is located. Although the terms “vacuum”and “negative pressure” may be used to describe the pressure applied tothe tissue site, the actual pressure applied to the tissue site may besignificantly greater than the pressure normally associated with acomplete vacuum. Consistent with this nomenclature, an increase inreduced pressure or vacuum pressure refers to a relative reduction ofabsolute pressure, while a decrease in reduced pressure or vacuumpressure refers to a relative increase of absolute pressure. The term“−Δp” means change in reduced pressure. As used herein, a greater (i.e.,more negative) −Δp means increased negative pressure (i.e., a greaterchange in pressure from ambient pressure). Reduced pressure treatmenttypically applies reduced pressure at −5 mm Hg to −500 mm Hg, moreusually −5 to −300 mm Hg, including but not limited to −50, −125 or −175mm Hg. Reduced pressure may be constant at a particular pressure levelor may be varied over time. For example, reduced pressure may be appliedand stopped periodically or ramped-up or -down overtime.

Referring to FIGS. 1-3 and the manifolds 120 and 220 collectively asmanifold 20 for ease of explanation, the systems 100 and 200 furthercomprise a scaffold structure 134 including one or more scaffold guidespositioned within the nerve gap 114 in fluid communication with themanifold 20 on one or both sides of the manifold 20 such as, forexample, scaffold guide 135. The scaffold guide 135 has a generallyfrusto-tubular shape with a base opening 136 at the base end of thefrustum and a vertex opening 137 at the other end of the frustum. Thescaffold guide 135 is positioned in the nerve gap 114 and orientedtherein so that the vertex opening 137 is positioned closer to themanifold 20 than the base opening 136 which faces the proximal segment104 when the vertex opening 137 is on the proximal side of the manifold20 or faces the distal segment 106 when the vertex opening 137 is on thedistal side of the manifold 20. More specifically, the scaffoldstructure 134 of the systems 100, 200 comprise six scaffold guides (onlyfour shown in FIGS. 1 and 2) including scaffold guides 135, 141, 143,145, 147, and 149 (collectively the “scaffold guides”) wherein thescaffold guides 135, 141, and 143 are positioned on the proximal side ofthe manifold 20 so that their vertex openings are closer to the manifold20 than their base openings which face the proximal segment 104. Asystem according to the invention may, however, comprise 1, 2, 3, 4, 5,6, 7, 8 or more scaffold guides. Correspondingly, the scaffold guides145, 147, and 149 are positioned on the distal side of the manifold 20so that their vertex openings are closer to the manifold 20 than theirbase openings which face the distal segment 106. The scaffold guides maybe formed of a scaffold fabric material or a web-like material 139 asillustrated by the concentric rings 139 a and ribs 139 b. In eitherembodiment, the scaffold guides function as nodes within the nerve gap114 for protein absorption and the initialization point for fibrilformation. The structure of the scaffold guides also wicks and directsslow-moving fluid within the nerve gap 114 from the base opening 136toward the vertex opening 137 and the source of the reduced pressure,i.e., the manifold 20. The scaffold guides may be composed of anybiocompatible material, but is preferably a bioabsorbable material.

The scaffold structure 134 may further comprise one or more fiber guides160 extending through the vertex openings of each of the scaffold guidesfrom the proximal segment 104 to the distal segment 106 of the severednerve 102. The fiber guides 160 may form a fiber bundle 159 alsoextending between the proximal segment 104 and the distal segment 106including, but not limited to, as many as one hundred fiber guides 160.The fiber guides 160 may also be fluidly and/or mechanically coupled tothe proximal segment 104 and/or the distal segment 106 of the severednerve 102. Additionally, the fiber guide 160 may be fluidly and/ormechanically coupled to the manifold 20. When fibril formation commencesas described above, the scaffold guides wick and direct fluid toward thevertex openings of the scaffold guides and the fiber guides 160 whichfacilitate fibril formation between the scaffold guides and ultimatelyextending between the base openings of the scaffold guides. Referringmore specifically to FIG. 3, fibril formation commences with directfluid flow toward the vertex openings of the scaffold guides and alongthe fiber guides 160 as indicated by the fibers 151 that grow along thepath created by the fiber guides 160. The fibers 151 may constituteeither provisional matrix fibers such as but not limited to fibrin,collagens, proteoglycans, and laminin, or cellular based structures suchas nerve fibers or supportive cell types. The provisional matrix andcellular based fibers may be derived either from endogenous hostsources, or from the introduction of exogenous fluids. As fibrilformation increases the density of the fibers 151, fibril formationexpands outwardly between the base openings of the scaffold guides asindicated by fibers 153. Ultimately, fibers 155 begin forming betweenthe scaffold guides having vertex openings facing each other, e.g.,scaffold guides 135 and 145. The fiber guides 160 act to guide cellmigration and growth through the entire scaffold structure 134 and maybe composed of any biocompatible material such as a bioabsorbablematerial. In certain cases, the fiber guides are composed of abiological material such as collagen or fibrin.

Referring now to FIGS. 3 and 4, three embodiments of a fiber guide 160with fluid flowing in a direction indicated by the dashed arrows 161 areshown including fiber guides 162, 164, and 166. Each of the fiber guides162, 164, and 166 comprise a strand of fiber material includingprotrusions for binding of proteins and cells from the slow-moving fluidwithin the nerve gap 114 to facilitate fibril formation. Morespecifically, fiber guides 162 and 164 include small barbs 163 and 165extending from the strands of the fibril guides 162 and 164 in adirection with fluid flow and against fluid flow, respectively,depending upon the fluidics within the nerve gap 114. Alternatively, thefiber guide 166 includes protrusions in the shape of hooks 167 tofacilitate protein binding and initiation sites of fibril formationthrough the nerve gap 114 without being aligned against fluid flow asare the small barbs 165 of the fiber guide 164. The the small barbs 163and 165 and the hooks 167 may also be oriented in a direction extendingeither toward the proximal segment 104 or the distal segment 106 of thesevered nerve 102 as may be required by the fluidics within the nervegap 114. In general, the fiber guides 162, 164, and 166 including theirthe small barbs 163 and 165 and hooks 167, respectively, are composed ofthe same materials described for use in other scaffold materials suchas, for example, collagen or fibrin. Referring to FIG. 5, a fiber guide160 may comprise a strand of fiber material that has a form other thanthe linear form described above and shown in FIG. 4. For example, thestrand of a fiber guide 168 is shaped in the form of a spiral having alongitudinal axis substantially parallel to the flow of fluid throughthe nerve gap 114 as indicated by dashed arrow 161. The fiber guide 168may also have protrusions extending from the strand such as, forexample, barbs 169 which are similar to the small barbs 163 of the fiberguide 162.

The nerve conduit 110 is shown in FIGS. 1 and 2 with a section removedto show the manifolds 120, 220 and is shown as completely surroundingthe nerve damage sites 108 as a closed nerve conduit 410 in FIG. 6.After the manifolds 120, 220 are inserted in the nerve damage sites 108,the closed nerve conduit 410 may be sealed by utilizing one or morestitches 415 or any other fastening device known in the art. The closednerve conduit 410 may be composed of a bioabsorbable or a bioinertmaterial. Materials that may be used for nerve conduits include, withoutlimitation, polylactic acid (PLA), polyglycolic acid (PGA),polylactide-co-glycolide (PLGA), polyvinylpyrrolidone, polycaprolactone,polycarbonates, polyfumarates, caprolactones, polyamides,polysaccharides (including alginates (e.g., calcium alginate) andchitosan), hyaluronic acid, polyhydroxybutyrate, polyhydroxyvalerate,polydioxanone, polyorthoesthers, polyethylene glycols, poloxamers,polyphosphazenes, polyanhydrides, polyamino acids, polyacetals,polycyanoacrylates, polyurethanes, polyacrylates, ethylene-vinyl acetatepolymers and other acyl substituted cellulose acetates and derivativesthereof, polystyrenes, polyvinyl chloride, polyvinyl fluoride,poly(vinylimidazole), chlorosulphonated polyolefins, polyethylene oxide,polyvinyl alcohol, Teflon®, and nylon. In certain aspects, biological(e.g., purified or recombinant) materials may be used to form nerveconduits including but not limited to, fibrin, fibronectin or collagen(e.g., DURAMATRIX™).

A nerve conduit 110, 410 may be an unbroken substantially tubularstructure fitted across a gap between a proximal and distal nerve stumpsuch as depicted in FIG. 6. Examples of such substantially tubular nerveconduits, also referred to as nerve guides, include without limitationNEURAGEN® and NEUROFLEX™ collagen conduits. A nerve conduit may also beformed from a wrap that is positioned around a disconnected or damaged(e.g., pinched) nerve and sealed with a closure, such as a suture.Specific examples of wrap-type nerve conduits include, withoutlimitation, NEUROMEND™ and NEURAWRAP™ collagen conduits. In certainaspects, the nerve conduit is made of a material that encloses thedamaged nerve, so as to exclude infiltration of non-nerve cells such asglia. In some embodiments, the nerve conduit material is permeable,thereby allowing fluid and protein factors to diffuse through theconduit. For example, the pores in a nerve conduit may be sufficientlysmall so as to exclude the entry of cells into the conduit lumen (e.g.,pores having an interior diameter or average interior diameter ofbetween about 5 μm and 50 μm, 10 μm and 30 μm or 10 μm and 20 μm). Thus,when reduced pressure is applied to the interior of the conduit fluidand proteins may be drawn to the lumen of the conduit by the pressuregradient. The skilled artisan will recognize that the dimensions of theconduit may be adjusted for any particular nerve application. Generally,the conduits comprise an internal diameter of less than about 6.0 mm,such as about 5, 4, 3, 2.5 or 2 mm.

Referring to FIG. 7, the reduced-pressure systems 100, 200, 700 mayfurther comprise a pressure sensor 740 operably connected to the firstconduit 125 to measure the reduced pressure being applied to themanifolds 120, 220. The systems 100, 200, 700 may further include acontrol unit 745 electrically connected to the pressure sensor 740 andthe reduced pressure source 115. The pressure sensor 740 measures thereduced pressure within the nerve gap 114 and also may indicate whetherthe first conduit 125 is occluded with blood or other bodily fluids. Thepressure sensor 740 also provides feedback to control unit 745 whichregulates the reduced pressure therapy being applied by the reducedpressure source 115 through the first conduit 125 to the manifolds 120,220. The reduced-pressure systems 100, 200, 700 may also comprise afluid supply 750 fluidly coupled to the first conduit 125 via a secondconduit 752 and operatively connected to the control unit 745. The fluidsupply 750 may be used to deliver growth and/or healing agents to thenerve damage site 108 including, without limitation, an antibacterialagent, an antiviral agent, a cell-growth promotion agent, an irrigationfluid, antibodies or other chemically active agents. The systems 100,200, 700 further comprises a first valve 754 positioned in the secondconduit 752 to control the flow of fluid therethrough, and a secondvalve 756 positioned in the first conduit 125 between the reducedpressure source 115 and the juncture between the first conduit 125 andthe second conduit 752 to control the flow of reduced pressure. Thecontrol unit 745 is operatively connected to the first and second valves754, 756 to control the delivery of reduced pressure and/or fluid fromthe fluid supply 750, respectively, to the manifolds 120, 220 asrequired by the particular therapy being administered to the patient.The fluid supply 150 may deliver the liquids as indicated above, but mayalso deliver air to the manifolds 120, 220 to promote healing andfacilitate drainage at the site of the nerve damage site 108.

As used herein, the term “manifold” refers to a substance or structurethat is provided to assist in directing reduced pressure to, deliveringfluids to, or removing fluids from a tissue site. A manifold can includea plurality of flow channels or pathways that are interconnected toimprove distribution of fluids provided to and removed from the area oftissue around the manifold. Examples of manifolds may include, withoutlimitation, devices that have structural elements arranged to form flowchannels, cellular foams such as open-cell foam, porous tissuecollections, and liquids, gels and foams that include or cure to includeflow channels. A detailed description of manifolds and their useaccording to the invention is provided below.

The term “scaffold” as used herein refers to a substance or structureapplied to or in a wound or defect that provides a structural matrix forthe growth of cells and/or the formation of tissue. A scaffold is oftena three dimensional porous structure. The scaffold can be infused with,coated with, or comprised of cells, growth factors, extracellular matrixcomponents, nutrients, integrins, or other substances to promote cellgrowth. A scaffold can take on characteristics of a manifold bydirecting flow through the matrix. A manifold can transmit flow to thescaffold and tissue; in the context of reduced pressure treatment, themanifold can be in fluid communication with the scaffold. A detaileddescription of scaffolds and their use according to the invention isprovided below.

As such, the invention disclosed here discloses methods and apparatusesfor controlling cellular-level based patterns of fluid flow that wouldallow for control of patterned protein organization at a microscopic,nanoscopic, or mesoscopic scale amenable to provide a structuredmanifold and, optionally, a scaffold material for cellular migration,differentiation, and like behavior necessary for functional regenerationof tissues. In comparison to the passive nature of the current state ofthe art with regards to tissue repair and regeneration, the methods,scaffolds, manifolds, flow sources and systems disclosed herein providean active mechanism by which to promote the endogenous deposition ofproteins and organization of the provisional matrix with biochemical andphysical cues to direct cellular colonization of a scaffold or tissuespace. The present invention thus enhances current technology byexploiting the active force of directed fluid flow, providing aframework upon which to design manifolds and scaffolds based upon theneed of the biology under the influence of fluid flow. Flow vectors andpathways are utilized to enhance protein deposition and cellularcolonization. The systems provided herein are designed to promoteestablishment of a provisional matrix network with a seamless transitionfrom the healthy tissue edges through a scaffold or tissue site topromote a functional tissue continuum.

Thus, the apparatuses, methods and systems disclosed herein provide ameans for active guidance of tissue regeneration through an implantedscaffold or within a tissue site to promote functional recovery. Thisactive guidance occurs through mechanisms of controlled fluid flow,which can be used to initiate or augment the early stages of the body'sown natural healing process; a manifold can provide the active guidancenecessary to create a controlled fluid flow. Specifically, thecontrolled flow vectors that the manifolds provide can be used tofacilitate the directed influx of cells and proteins into a scaffold.Creation of specific flow pathways within a tissue site or scaffold canlead to patterned deposition of proteins, such as collagen and fibrinwithin the manifold, scaffold or tissue space. Biochemical cues fromcytokines, growth factors, and cells bound within the provisional matrixcan work in conjunction with the natural physical cues of theprovisional matrix and extracellular matrix to guide the subsequentmigration of endogenous cells during the repair stages of healing. Thesecues act to establish a biological continuum that emanates from thehealthy tissues and passes through the scaffolding or tissue space tofacilitate a continuous guidance pathway for organized tissueregeneration.

To that end, this disclosure provides unique manifolding technologiesdesigned for specific biological needs based upon principles of fluidflow. In certain aspects, the invention concerns a new approach to woundhealing, flow (or gradient) activated tissue engineering. In rudimentaryform, this approach involves a source or generator of flow that forms agradient for controlled movement of either endogenous or exogenousfluids into, out of, or through a tissue space for the organizeddeposition of proteins and/or spatial concentration of cytokines andgrowth factors, with subsequent formation of a directionally orientedprovisional matrix. The tissue space being defined here includes, but isnot limited to, the region surrounding a site of nerve tissue damage.

Fluid flow into, through, or out of the nerve tissue space can berefined and directed through the inclusion of additional elements to thesystem including manifolds and/or scaffolds. The coordinated elements ofthe system are designed to create flow parameters, pathways, andpatterns sufficiently detailed in scale as to be able to influence anddirect the controlled adsorption of proteins, the organization ofmatrix, and organized colonization of specific cell types. Individualelements of the system are as follows.

Source or Generator of Flow.

Flow is induced into, through, or out of the nerve tissue space bymethods or apparatuses that introduce changes in mechanical, chemical,and/or electrical potentials. These generators of flow provide either agradient or a change in potential from the site or reservoir ofendogenous or exogenous fluids to the placement position of the flowgenerator or its extension element (i.e., manifold or scaffold). In oneembodiment, the source of flow comprises a source of reduced pressure.Systems and apparatuses according to the invention may also comprisevalves or arrays of valves that control the application and amount ofnegative pressure applied to the manifold. In certain aspects, nerveconduits and/or manifolds described herein comprise a pressure sensor.Thus, in some embodiments, the amount of negative pressure applied by asource is regulated based on the amount of negative pressure that issensed in the manifold or nerve conduit or at the site of tissue damage.

Manifold.

The flow generators are the driving force for stimulating the flow offluids. Manifolds are apparatuses for refining the pattern of flowbetween the source or generator of flow and the tissue space. Themacroscale level of flow is refined by specialized manifolds utilizedfor directed localization to a single point or to a plurality ofselectively positioned points for creating initiation sites formicroscale flow pathways within the manifold/scaffold and, ultimately,the tissue space. The manifold may also serve as a conduit for theremoval of fluids from and as an apparatus for the delivery of exogenousfluids to the tissue space.

A manifold generally refers to a physical substance or structure thatserves to assist in applying and translating a mechanical, chemical,electrical or similar alterations into changes in the flow of a fluid,herein defined as the movement of liquids, gases, and other deformablesubstances such as proteins, cells, and other like moieties. As such;this physical device includes a single point or plurality of points forthe egress or evacuation of pressure, fluids, and like substancescapable of translating the movement of fluids in a scaffold, as definedabove. This can include, but is not limited to, the introduction ofexogenous factors such as cells and/or therapeutic moieties into thescaffold through the lumen or plurality of lumens present in themanifold. In addition, as used herein, a manifold includes a singlepoint or plurality of points for the ingress or introduction of fluidfrom the scaffold back toward the point source of flow.

Flow distributed by the manifold can direct the movement of endogenousproteins, growth factors, cytokines, and cells from their residentlocations within the host to the tissue space or scaffold in anorganized manner. The establishment of flow along these pathways leadsto the deposition of proteins and provisional matrix that creates aninterfacial endogenous network connecting the host to the scaffold.Extensions of this matrix can be established within the scaffold throughselective positioning of the manifold flow initiation sites with flowpromoting scaffolding designs. The organized protein deposition andprovisional matrix provide a biochemical and physical framework thatstimulates the attachment and migration of cells along directed pathwaysthroughout the scaffold and the tissue space. The resulting endogenousnetwork of proteins, growth factors, and cells provides a foundationupon which subsequent phases of the body's own tissue repair andregeneration mechanisms can build.

When in place, the manifold works in conjunction with a flow generatingsource and a scaffold, if present. Flow generating sources include, butare not limited to generators of negative pressure; generators ofpositive pressure; and generators of osmotic flow. The flow gradientestablished in the manifold may be further refined through the scaffold,to deliver a flow gradient to the scaffold to optimize flow through thescaffold as needed for the particular defect. Many of the embodimentsdisclosed herein are manifolds capable of translating changes inpressure and the like into controlled movement of fluids, optionallythrough a physical scaffold, for the purposes of directed tissueregeneration. These embodiments are generally specified for a particularapplication in the regeneration of specific tissues, but are not limitedto a particular tissue therein.

In order to realize the goal of inducing flow for the purpose of tissueregeneration, alterations in the aforementioned mechanical, chemical, orelectrical impetus must be translated from the singular gradient sourcetoward a physical substrate or scaffold to elicit cellular-level changesin protein adsorption, matrix organization, cell migration, and othertissue regeneration-related behaviors. These alterations aremultivariate in nature and can include mechanical changes that elicit aphysical change in pressure applied to the scaffold as applied to thesite of the wound or desired site of tissue regeneration, chemicalchanges that elicit a gradient in protein and/or ion concentrations,which result in the creation of osmotic gradients capable of inducingflow, or electrical changes that create a gradient of current/ionexchange allowing for propagation of electrical signals from the pointsource. It is to be understood, however, that the applicants are notbound by any particular mechanism through which gradients and fluid flowinduce advantageous results in tissue repair or growth. In order toadvantageously transmit these gradients to the tissue, a physical deviceis needed to direct the path of flow from its source to the scaffold ortissue site and vice versa.

In some embodiments, the manifold comprises a physical structure inclose apposition to or within the contents of a scaffold and serves topropagate an alteration in a physical parameter, whether it bemechanical, chemical, electrical, or something similar in nature, forthe means of directing these changes from its point source to thescaffolding material. The placement of this manifold with respect to itslocation with regard to that of the scaffold may be of crucialimportance for facilitating controlled and directed regeneration ofspecific tissue types. For example, in peripheral nerve whereregeneration primarily occurs in a unidirectional manner from theproximal to distal nerve stumps, it may be important to place themanifold along the length of a nerve conduit more toward it distal endto help direct regeneration toward that end. However, it may also beimportant to not place the manifold at the most distal aspect of thescaffold/conduit as soluble factors derived from the distal stump havebeen shown to be important for directing nerve regeneration toward itssource.

Manifolds may be composed of a bioabsorbable or bioinert material.Examples include non-bioabsorbable materials such as medical gradesilicone polymers, metals, polyvinylchloride (PVC), and polyurethane.Bioabsorbable polymers such as collagen, polylactic acid (PLA),polyglycolic acid (PGA), polylactide-co-glycolide (PLGA), apolysaccharide, a hydroxyapatite, or a polyethylene glycol, orcombinations thereof, can also be used. Some manifolds are also a mix ofnon-bioresorbable and bioresorbable materials. In general material usedfor a scaffold may also be used to compose a manifold and such materialsare further detailed below. In certain aspects, manifold materials arestructured to include a high void fraction for improved bioabsorptionproperties.

Support.

Manifold support structures, such as a manifold chamber and/or flangemay be composed of any acceptable biocompatible material. A supportstructure will typically be impermeable and surround the manifold so asto maintain manifold pressure.

A portion of the support, such as a flange, couples the manifold and thenerve conduit. In certain aspects, a flange is attached to the exteriorsurface of a nerve conduit with an adhesive such as a fibrin glue,cyanoacrylate, or other biologically derived adhesive. A support mayalso be connected to a nerve conduit via reversible mechanisms otherthan an adhesive, such as chemical, thermal, osmotic, mechanical (snapor other interference fit, threaded, etc), magnetic, and electrostaticmechanisms. The manifold may be used to deliver agents that reverse theaction of the binding mechanism in order to detach the support from thenerve conduit (e.g., upon completion of therapy). For example,electrostatic binding may be released through introduction of saltsolutions or biocompatible solvents may be used to release adhesives.

Scaffold.

Biologic and synthetic scaffolds are used in the field of tissueengineering to support protein adhesion and cellular ingrowth for tissuerepair and regeneration. The current state of the art in scaffoldtechnology relies upon the inherent characteristics of the surroundingtissue space for the adsorption of proteins and migration of cells. Ascaffold for use according to the invention is coupled to a manifold,provides physical guidance to direct the pathway of fluid flow in thetissue site, creating avenues for the movement and migration of adhesiveproteins and cells, respectively, which are integral to theestablishment of a provisional matrix in predetermined patterns oforganization within the tissue space. The methods and apparatusesdescribed for fluid flow-induced and gradient-induced generation oftissues have direct implications into the design of the scaffolds.Within this context, scaffolds serve to refine the pathways of fluidflow within the tissue space to cellular level patterns from the fluidsource to the point(s) of flow initiation within the manifold. Ascaffold may embody characteristics of a manifold or be combined inconjunction with a manifold for refinement of the flow pathways withinthe tissue site. In certain aspects, a scaffold is a reticulatedstructure comprising high void fraction for improved bioabsorptionproperties.

Scaffolds may also comprise retention structures as described hereinsuch as funnel guides and fiber guides. For example, funnel guides maybe used to direct the diffusion and/or migration of cells or growthfactors at a site of nerve damage. In some cases, multiple funnel guidessuch as 2, 3, 4, 5, 6, 7, 8 or more funnel guides are comprised in ascaffold. A funnel guide may be composed of a hydrophobic material andmay be bioabsorbable so as to degrade as nerve tissue grows into thesite of the nerve damage. Funnel guides may also be hydrophilic toassist in the directed movement of slow moving fluids e.g., by wicking.Funnel guides may also have bioabsorption properties that differ fromthe narrow to the wide end of the funnel guide. For instance, the narrowend of the funnel guide may be absorbed at a faster rate so that theaperture of the narrow end becomes wider as tissue regrowth occurs.Likewise, in aspects where multiple funnel guides are comprised in ascaffold, funnel guides closer to the proximal end of the nerve damagesite may be composed of a material that is absorbed at a faster rate sothat funnel structures closer to the proximal end of a nerve damage siteare absorbed more rapidly.

Fiber guides in scaffolds may also be composed of bioabsorbable materialsuch that the guides are absorbed as tissue growth or regrowth occurs.As detailed above, fiber guides may comprise protrusions (e.g., barbs)or hook structures and may be essentially linear or form a spiral asthey extend through the scaffold at the nerve damage site. In certainaspects, the fiber guides and associated structures (e.g., fiberprotrusions and hooks) direct cell growth or migration at the site ofnerve damage. In some embodiments, the fiber structures comprisebioactive molecules as part of their structure. For example, fiberstructures may comprise growth factors that enhance cell growth alongthe length of the fibers or binding moieties (such as antibodies) thatbind cells or growth factors to the fibers to enhance the growth orregrowth of nerve tissue.

Nonlimiting examples of suitable scaffold, funnel and fiber materialsinclude extracellular matrix proteins such as fibrin, collagen orfibronectin, and synthetic or naturally occurring polymers, includingbioabsorbable or non-bioabsorbable polymers, such as polylactic acid(PLA), polyglycolic acid (PGA), polylactide-co-glycolide (PLGA),polyvinylpyrrolidone, polycaprolactone, polycarbonates, polyfumarates,caprolactones, polyamides, polysaccharides (including alginates (e.g.,calcium alginate) and chitosan), hyaluronic acid, polyhydroxybutyrate,polyhydroxyvalerate, polydioxanone, polyethylene glycols, poloxamers,polyphosphazenes, polyanhydrides, polyamino acids, polyortho esters,polyacetals, polycyanoacrylates, polyurethanes, polyacrylates,ethylene-vinyl acetate polymers and other acyl substituted celluloseacetates and derivatives thereof, polystyrenes, polyvinyl chloride,polyvinyl fluoride, poly(vinylimidazole), chlorosulphonated polyolefins,polyethylene oxide, polyvinyl alcohol, Teflon®, and nylon. The scaffoldcan also comprise ceramics such as hydroxyapatite, corallin apatite,calcium phosphate, calcium sulfate, calcium carbonate or othercarbonates, bioglass, allografts, autografts, xenografts, decellularizedtissues, or composites of any of the above. In particular embodiments,the scaffold comprises collagen, polylactic acid (PLA), polyglycolicacid (PGA), polylactide-co-glycolide (PLGA), a polyurethane, apolysaccharide, an hydroxyapatite, or a polytherylene glycol.Additionally, the scaffold can comprise combinations of any two, threeor more materials, either in separate areas of the scaffold, or combinednoncovalently, or covalently (e.g., copolymers such as a polyethyleneoxide-polypropylene glycol block copolymers, or terpolymers), orcombinations thereof. Suitable matrix materials are discussed in, forexample, Ma and Elisseeff, 2005, and Saltzman, 2004.

Bioactive Agents

In certain aspects, the apparatuses and methods according to theinvention concern bioactive agents. Bioactive agents may, in some cases,be incorporated directly onto a manifold or scaffold material (i.e., togenerate a bioactive manifold and/or scaffold). For example, agents thatfacilitate tissue growth such as collagen or fibrin may be directlyincorporated onto or into a manifold or scaffold material. Likewise, inapplications where aberrant immune response need be avoided (e.g.,tissue grafts) immune regulator agents such as rapamycin may beincorporated into manifold or scaffold structures.

In further aspects soluble bioactive agents may be introduced at a siteof tissue damage by virtue of flow through the tissue site. For example,a manifold may be in fluid communication with a fluid source and abioactive agent may be introduced into the fluid source and thereby intothe manifold and damaged nerve tissue.

Nonlimiting examples of useful bioactive growth factors for variousapplications are growth hormone (GH), a bone morphogenetic protein(BMP), transforming growth factor-α(TGF-α), a TGF-β, a fibroblast growthfactor (FGF), granulocyte-colony stimulating factor (G-CSF),granulocyte/macrophage-colony stimulating factor (GM-CSF), epidermalgrowth factor (EGF), platelet derived growth factor (PDGF), insulin-likegrowth factor (IGF), vascular endothelial growth factor (VEGF),hepatocyte growth factor/scatter factor (HGF/SF), an interleukin, tumornecrosis factor-α(TNF-α) or nerve growth factor (NGF).

Nerve Tissue Repair and Engineering.

The apparatuses and systems disclosed herein can be used for nervetissue repair and engineering in various contexts including thefollowing.

Repair and Regeneration of Lost Tissue.

A generator of flow may be combined with manifolds and/or scaffolds todirect the regeneration of lost tissue at a site of injury orcompromised function. Tissues lost from traumatic injury, surgery,burns, or other causes (e.g., infection or autoimmune disease) can beled to regenerate using the methods, scaffolds, manifolds, flow sourcesand systems of the invention. Functional nerve tissue is directed toregenerate.

Retard the Progression of a Tissue Disease State.

A generator of flow may be combined with manifolds and/or scaffolds toretard disease progression of an affected nerve tissue such as occurs,e.g., in autoimmune disease.

Maintenance of Tissue Viability.

A generator of flow may be combined with manifolds and/or scaffolds tomaintain the viability of explanted tissues either for in vitro study,ex vivo scaffold or implant preparation, or in vivo transplant. Agenerator of flow combined with a manifold may be used to provide fluidflow to the tissue and to control waste removal from the tissue.

Expansion of Tissue.

A generator of flow may be combined with manifolds and/or scaffolds topromote the expansion of existing tissues. The methods, scaffolds,manifolds, flow sources and systems of the invention can be used todirect the growth of tissues where additional tissue quantity is neededor desired

Acceleration of Tissue Formation.

A generator of flow may be combined with manifolds and/or scaffolds toaccelerate the rate of tissue formation within a natural healingresponse. The methods, scaffolds, manifolds, flow sources and systems ofthe invention may be used to accelerate tissue growth by augmentingformation of provisional matrices, facilitating its stable positioning,and aiding in recruitment of cells to the tissue space.

Stimulating Differentiation of Stem Cells Along Specific Pathways.

A generator of flow may be combined with manifolds and/or scaffolds tostimulate differentiation of stem cells or other pluripotent cells intospecific lineages. Application of flow using the methods, scaffolds,manifolds, flow sources and systems of the invention may be used todirect pluripotent cells into specific cell lineages needed to fostergrowth in the tissue space.

Introducing Proteins, Matrix, Cells, or Pharmaceuticals into the In VivoEnvironment.

A generator of flow may be combined with manifolds and/or scaffolds tointroduce exogenous growth factors, proteins, cells, or pharmaceuticalagents into the tissue space to augment tissue repair, regeneration,and/or maintenance.

Creating Matrices In Vitro for Implantation In Vivo.

A generator of flow may be combined with manifolds and/or scaffolds tofacilitate formation of matrices in vitro that may subsequently be usedfor in vivo transplantation.

Promoting Integration of Transplanted Tissue.

A generator of flow may be combined with manifolds and/or scaffolds topromote integration of transplanted tissue into the host environment.This can be applied to autograft, allograft, or xenograft transplants.

Directing Extracellular Matrix (ECM) Deposition and Orientation InVitro.

A flow generator may be combined with manifolds and/or scaffolds toguide the directed deposition and orientation of ECM expressed by cellsand tissues. The directed orientation of ECM has an impact in organizingand directing the attachment and colonization of subsequent cell layersand tissues.

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All references cited in this specification are hereby incorporated byreference. The discussion of the references herein is intended merely tosummarize the assertions made by the authors and no admission is madethat any reference constitutes prior art. Applicants reserve the rightto challenge the accuracy and pertinence of the cited references.

In view of the above, it will be seen that the advantages of theinvention are achieved and other advantages attained. As various changescould be made in the above methods and compositions without departingfrom the scope of the invention, it is intended that all mattercontained in the above description and shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

We claim:
 1. A system for providing reduced pressure to a defect at atissue site of a nerve, the system comprising: a pressure source forsupplying the reduced pressure; a nerve conduit having a generallytubular shape and having a luminal wall for surrounding the tissue siteto contain fluids within a luminal space between the tissue site and theluminal wall; a manifold fluidly coupled to the pressure source todistribute the reduced pressure to the defect; and a scaffold comprisingat least one scaffold guide having a frustum-shaped conical tubularmember with a base opening at one end and a vertex opening at the otherend wherein the at least one scaffold guide is positioned within theluminal space in fluid communication with the manifold and orientedtherein with the vertex opening positioned closer to the manifold thanthe base opening, and at least one fiber guide extending longitudinallythrough the luminal space and the vertex opening of the at least onescaffold guide; whereby the at least one scaffold guide facilitatestissue growth at the tissue site by organizing matrix deposition,promoting cell binding, or guiding the migration of cells through theluminal space.
 2. The system of claim 1, wherein the defect is a severednerve.
 3. The system of claim 1, wherein the defect is a pinched,partially severed, or degenerative nerve and the at least one scaffoldguide is adapted to receive the nerve through the vertex opening of theat least one scaffold guide.
 4. The system of claim 1, wherein the atleast one scaffold guide is substantially non-porous.
 5. The system ofclaim 1, wherein the at least one scaffold guide comprises pores thatare sufficiently small to collect cells.
 6. The system of claim 1,wherein the at least one scaffold guide is composed of a hydrophobicmaterial.
 7. The system of claim 1, wherein the at least one fiber guideis adapted to be in fluid communication with the tissue site of thenerve.
 8. The system of claim 1, wherein the at least one fiber guide isin fluid communication with the manifold and the at least one scaffoldguide.
 9. The system of claim 1, wherein the at least one fiber guidecomprises fibrous protrusions extending into the luminal space.
 10. Thesystem of claim 1, wherein the at least one fiber guide is linear. 11.The system of claim 1, wherein the at least one fiber guide isbioabsorbable.
 12. The system of claim 11, wherein the at least onefiber guide is comprised of collagen or fibrin.
 13. The system of claim1, wherein the manifold is adapted to be positioned adjacent a distalside of the nerve.
 14. The system of claim 1, wherein the manifoldprovides reduced pressure preferentially to a distal side of the nerve.15. The system of claim 1, wherein the manifold is composed of abioinert or bioabsorbable material.
 16. The system of claim 1, whereinthe manifold is substantially tubular or cylindrical in shape andpositioned within the luminal space of the nerve conduit.
 17. The systemof claim 1, wherein the manifold extends through the wall of the nerveconduit in fluid communication with the luminal space.
 18. The system ofclaim 1, wherein the scaffold is formed from a foam or gel material. 19.The system of claim 1, wherein the scaffold comprises a bioactive agent.20. The system of claim 19, wherein the bioactive agent is at least oneof an antibiotic, an antibody and a growth factor.
 21. The system ofclaim 19, wherein the bioactive agent is a growth hormone (GH), a bonemorphogenetic protein (BMP), transforming growth factor-α (TGF-α), aTGF-β, a fibroblast growth factor (FGF), granulocyte-colony stimulatingfactor (G-CSF), granulocyte/macrophage-colony stimulating factor(GM-CSF), epidermal growth factor (EGF), platelet derived growth factor(PDGF), insulin-like growth factor (IGF), vascular endothelial growthfactor (VEGF), hepatocyte growth factor/scatter factor (HGF/SF), aninterleukin, tumor necrosis factor-α (TNF-α) or nerve growth factor(NGF).
 22. The system of claim 1, wherein the nerve conduit comprises aslice along its length that forms an opening whereby the nerve conduitis implantable around the tissue site and sealable with one or moreclosure elements.
 23. The system of claim 1, wherein the nerve conduitis composed of a bioinert material.
 24. The system of claim 1, whereinthe nerve conduit is composed of a bioabsorbable material.
 25. Thesystem of claim 24, wherein the nerve conduit is composed of collagen.26. The system of claim 1, wherein the nerve conduit comprises pores.27. The system of claim 26, wherein the pores are sufficiently small toexclude the entry of cells from tissue surrounding the nerve conduitinto the luminal space.
 28. The system of claim 27, wherein the poreshave a diameter of between about 5 μm and about 50 μM.